1. Field of the Invention
The invention relates to an intermediate frequency circuit for a television tuner, in which, when the intermediate frequency circuit for the television tuner is formed on a circuit board, the coupling state of components disposed between the input and output of an intermediate frequency amplifier is selected, and a picture portion in a frequency characteristic of the intermediate frequency amplifier is set to become a peak.
2. Description of the Related Art
In general, an intermediate frequency circuit for a television tuner formed on a circuit board includes an intermediate frequency tuning circuit, which has a coil and a capacitor connected parallel to each other and resonates parallel to an intermediate frequency signal, an N−1 channel trap circuit, which is connected to the output of the intermediate frequency tuning circuit and has a coil and a capacitor connected parallel to each other, for removing voice signal components of lower neighboring channels, an intermediate frequency amplifier connected to the output of the N−1 channel trap circuit, for amplifying the intermediate frequency signal, and an intermediate frequency signal resonance circuit connected to the output of the intermediate frequency amplifier and having a coil and a capacitor connected in series. Furthermore, in the intermediate frequency circuit for the television tuner, in order to avoid induction coupling of the coil constituting the N−1 channel trap circuit and the coil constituting the intermediate frequency signal resonance circuit as much as possible, the two coils are disposed on the circuit board so as to be orthogonal to each other.
In this case, FIG. 7 shows an exemplary construction of an intermediate frequency circuit for a television tuner according to a related art, including a front part circuit. FIG. 8 is a plan view illustrating the disposition state of main components when the intermediate frequency circuit for the television tuner shown in FIG. 7 is formed on the circuit board. In addition, the same reference numerals as in the components shown in FIG. 7 denote the same components of FIG. 8 in the description below.
As shown in FIG. 7, the intermediate frequency circuit 50 of the television tuner includes an intermediate frequency tuning circuit 51 in which two coils 51(1) and 51(2) that are connected in series are connected parallel to a capacitor 51(3), a N−1 channel trap circuit 52 in which a coil 52(1) is connected parallel to a capacitor 52(2), an intermediate frequency signal amplifier 53, an intermediate frequency signal resonance circuit 54 in which a coil 54(1) and a capacitor 54(2) are connected in series, a frequency converter 55 including a mixer circuit and a local oscillation circuit, a buffer resistor 56, a bypass capacitor 57, a coupling capacitor 58, a bypass capacitor 59, a shunt capacitor 60, an input terminal 50(I), an output terminal 50(O), and a power supply terminal 50(B). These components 51 to 60 are interconnected, as shown in FIG. 7. Furthermore, the front part circuit of the intermediate frequency circuit 50 of the television tuner includes a high frequency circuit 61 including a bandpass filter, a high frequency amplifier, a television signal select filter or the like, and a reception antenna 62 which is connected to the high frequency circuit 61 as shown in FIG. 7.
Furthermore, as shown in FIG. 8, in the disposition state of the components when the intermediate frequency circuit 50 of the television tuner is formed on the circuit board, an integrated circuit block 53,55 of the intermediate frequency signal amplifier 53 and the frequency converter 55 is disposed at the central region of the board. The integrated circuit block 53,55 has two mixer output terminals (Mix out) and two intermediate frequency input terminals (IF in) that are derived from one side, and an intermediate frequency output terminal (IF out) that is derived from another side. The two coils 51(1) and 51(2) and the coil 52(1) are disposed parallel along one side of the integrated circuit block 53,55. The coil 54(1) is disposed so as to be orthogonal to the coil 52(1). Furthermore, the capacitor 51(3) is connected to ends of the two coils 51(1) and 51(2) therebetween and the other end being directly connected to each other by a lead, thus forming the intermediate frequency tuning circuit 51. The coil 52(1) has both ends being connected to the capacitor 52(2) therebetween, thus forming the N−1 channel trap circuit 52. The coil 54(1) is connected to the capacitor 54(2) in series, thus forming the intermediate frequency signal resonance circuit 54. In addition, the respective components 56 to 60 are connected, as shown in FIG. 8, and the intermediate frequency circuit 50 of the television tuner is provided.
As described above, in the intermediate frequency circuit for the television tuner according to the related art shown in FIGS. 7 and 8, the coil 52(1) of the N−1 channel trap circuit 52 and the coil 54(1) of the intermediate frequency signal resonance circuit 54 are disposed orthogonally. The two intermediate frequency input terminals (IF in) and the two intermediate frequency output terminals (IF out) in the integrated circuit block 53,55 are disposed on opposite sides to each other. Therefore, the feedback amount between the input and output of the intermediate frequency amplifier 53 is small.
In this case, FIGS. 9A and 9B show characteristics obtained in the intermediate frequency circuit for the television tuner according to the related art shown in FIGS. 7 and 8. FIG. 9A shows a frequency characteristic and FIG. 9B shows a video frequency characteristic.
As shown in FIG. 9A, the intermediate frequency circuit for the television tuner according to the related art has a characteristic in that in the frequency characteristic, a peak level is inclined toward a carrier (C) portion than a picture (P) portion and the level of the picture (P) portion is about 0.5 dB lower than the peak level.
Meanwhile, as shown in FIG. 9B, the intermediate frequency circuit for the television tuner according to the related art has a characteristic in that in the video frequency characteristic, there are a portion that rises near 1 MHz from a reference level of 0 MHz, and a portion that abruptly declines near 3.58 MHz.
As described above, the intermediate frequency circuit for the television tuner according to the related art has a construction in which coupling between the input and output side of the intermediate frequency amplifier becomes weak when the intermediate frequency circuit for the television tuner is formed on the circuit board. Thus, the peak level and the picture portion in the frequency characteristic are not identical to each other, or there occurs a rising portion or an abruptly declined portion in the video frequency characteristic, which are not satisfactory characteristics.